1. Field of the Invention
The present invention relates to a method and an apparatus for controlling a hydraulically operated anti-lock braking system for a motor vehicle, so as to avoid an excessive slip ratio of a wheel of the vehicle during brake application, and more particularly improvements in connection with an operation to lower the brake fluid pressure applied to a brake for the wheel.
2. Discussion of the Prior Art
A hydraulically operated brake is widely used for an automotive vehicle. The brake is actuated by a fluid pressure generated by a suitable hydraulic pressure source such as a master cylinder or a hydraulic pump, so that the brake applies a braking force to the corresponding wheel of the vehicle.
If the fluid pressure applied to the hydraulic brake is excessively high with respect to the coefficient of friction of a road surface, the rotating wheel tends to be locked and skids on the road surface. Even where the wheel is not perfectly locked, the friction force between the wheel surface and the road surface is reduced if the slip ratio of the wheel exceeds an optimum level. In this instance, the required braking distance of the vehicle is increased, or the running stability of the vehicle is deteriorated.
In light of the above undesirable phenomenon during brake application to the wheel, the hydraulic brake is controlled in a so-called "anti-lock" or "anti-skid" manner. An example of an anti-lock control arrangement is disclosed in Laid-open publication No. 58-26661 (published in 1983) of unexamined Japanese Patent Application. In the disclosed arrangement, a locking or skidding condition of the wheel as represented by a slip amount or slip ratio of the wheel is detected by comparing a running condition of the vehicle such as a running speed or deceleration rate of the vehicle, with a rotating condition of the wheel such as a rotating speed or deceleration rate of the vehicle. The slip ratio of the wheel is maintained within an optimum range, by increasing, decreasing or holding the brake fluid pressure, based on the detected locking condition of the wheel.
Such an anti-lock braking system is adapted to commence decreasing the brake fluid pressure if and when the slip amount or ratio of the wheel exceeds a predetermined value. When a predetermined pressure-decrease time has passed, the fluid pressure is held at the reduced level or increased. According to another control arrangement, the decrease in the fluid pressure is commenced if and when the deceleration rate of the wheel falls below a predetermined level. In this case, the fluid pressure is held at the reduced level or increased when the wheel deceleration rate is restored to the predetermined level. These are some examples of the manner in which the brake fluid pressure is regulated in a known anti-lock manner.
However, since the slipping or skidding condition of a vehicle wheel significantly varies depending upon various conditions or parameters when a brake is applied to the wheel, there still exists a problem to be solved. That is, the brake fluid pressure cannot be adequately decreased by an optimum amount if the fluid pressure is decreased for a predetermined fixed time span or as long as the deceleration rate of the wheel is below a predetermined level. Consequently, the known anti-lock control systems are not completely satisfactory, suffering from a relatively long braking distance of the vehicle and/or an insufficient degree of straight-line running stability of the vehicle.